Closure Assembly with Low Vapor Transmission for Electrochemical Cell

ABSTRACT

A closure assembly for an electrochemical cell including a container and an end assembly sealing an open end of the container in order to minimize mass or weight loss of the cell due to electrolyte vapor transmission is disclosed. The end assembly is provided with a vent member capable of venting a fluid when the pressure within the cell exceeds a predetermined limit; a contact member operatively in electrical contact with a conductive contact of the end assembly and a current collector of an electrode of the cell; and an insulating, polymeric seal member disposed at least between conductive components of the closure assembly having different polarities. In a preferred embodiment, the seal member has a selected dimensional ratio in order to minimize vapor transmission of the electrolyte through the seal member.

This application is a Divisional of U.S. Ser. No. 12/136,910, filed Jun.11, 2008, the disclosure of which is fully incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to a closure assembly for anelectrochemical cell including a container and an end assembly sealingan open end of the container in order to minimize mass or weight loss ofthe cell due to electrolyte vapor transmission. The end assembly isprovided with a vent member capable of venting a fluid when the pressurewithin the cell exceeds a predetermined limit; a contact memberoperatively in electrical contact with a conductive contact of the endassembly and a current collector of an electrode of the cell; and aninsulating, polymeric seal member disposed at least between conductivecomponents of the closure assembly having different polarities. In apreferred embodiment, the seal member has a selected dimensional ratioin order to minimize vapor transmission of the electrolyte through theseal member.

BACKGROUND OF THE INVENTION

Electrochemical cells, such as those containing lithium metal or alloyas an electrochemically active material, are utilized to provide powerto various electronic devices. Electronic device manufacturers oftendesign their devices to accept electrochemical cells having variousstandardized container exterior dimensions, such as “AA” or “AAA” sizes,or according to ANSI nomenclature, R6 or R03 size containers,respectively. Regulatory bodies such as the United Nations (UN) andDepartment of Transportation (DOT) have mandated requirements regardingtransportation of lithium-containing electrochemical cells. In additionto regulating the maximum lithium content for a certain cell type,UN/DOT regulations require that lithium-containing cells pass mass losstests, for example a T1 altitude simulation test and a T2 thermalcycling test.

Mass or weight loss in electrochemical cells can be attributed tosources such as diffusion of electrolyte vapor through a sealing memberof the cell and electrolyte leakage at sealing interfaces, especiallyduring temperature cycling. The diffusion weight loss can be calculatedin one embodiment as a product of the vapor transmission rate of thesealing member, the dimensional ratio of the sealing member, and time.The dimensional ratio can be calculated by dividing the cross sectionarea through which the electrolyte vapor diffuses with the path lengththat the vapor travels. As cell size decreases, the ratio of the crosssection area to the path length does not decrease as quickly as thevolume or the mass of the cell. Therefore, relatively small size cellstend to have higher percentage mass loss than larger cells, and it isbelieved to be more difficult for small size cells to pass UN/DOT massloss requirements.

Moreover, electrochemical cells such as electrochemically activelithium-containing cells often utilize a non-aqueous electrolytesolution and salt that can be volatile and/or reactive. In view thereof,it is a challenge to construct an electrochemical cell that minimizesmass loss due to vapor transmission.

A further challenge is to provide the cell with a pressure release ventmember for releasing or discharging fluid from inside the cell to limitthe build-up of internal pressure while maintaining a seal during normaldischarge or storage conditions. Without a vent member, the cell mayfail, bulge, leak and/or disassemble.

Various pressure release vent member and closure assembly configurationshave been used in electrochemical cells.

U.S. Pat. No. 3,279,953 relates to reportedly insulating seals for themetallic casing of sealed battery cells. Specifically, it relates to theinsulating seal junction between the open end of the tubular metallicsheet casing and the metallic sheet cover enclosure which alsoconstitutes the two opposite-polarity terminals of sealed cells, such asreportedly used, for example, in flashlights, although similar sealedcasings have also been used in other applications.

U.S. Pat. No. 3,852,117 relates to a seal for an electrochemical cell orthe like located between the cylinder wall and closure disc at one endof a cylindrical container. The seal comprises opposed circular sealingmembers formed by deformation of the cylinder wall, bearing againstopposite faces of the disc around its rim. The seal is closed by axialcompression of the cylinder wall causing deformation of the wall to formthe sealing members and to press such members against the closure disc.

U.S. Pat. No. 5,876,868 relates to a battery sealing structure with areportedly explosion-proof function of preventing battery explosion dueto an abnormal increase of the inner pressure in the battery and alsowhich reportedly is capable of excellently sealing the battery which maycontribute to improvements in battery assembly operation efficiency.

U.S. Pat. No. 6,207,320 relates to a battery including a can filled withan electrolyte and an electrode assembly. A cap assembly is reportedlyclose-tightly mounted on an upper end of the can with a gasketinterposed between the cap assembly and the upper end. The cap assemblyprovides a plate provided with a safety groove, a current control memberdisposed on the plate, a cap cover disposed on the current controlmember, and a circuit breaker disposed under the plate and supported bya support plate. Also, the circumferential edge of at least one of theplate, the current control member and the cap cover is bent around thesupport plate.

U.S. Pat. No. 6,620,544 relates to a sealed battery which includes a canfor receiving an electric generator, a sealing member crimped on anopening of the can and connected to one of a positive electrode and anegative electrode of the electric generator, a gasket disposed betweenthe can and the sealing member, a cover cap disposed on the sealingmember with an insulating member disposed between the cover cap andsealing member, a current control member disposed between the cover capand the sealing member to reportedly cut-off a flow of current when atemperature of the battery is increased above an allowable level, and ashock absorber disposed between the cover cap and current control memberto reportedly prevent shock from being directly transmitted to thecurrent control member.

U.S. Pat. No. 6,777,128 relates to a secondary battery and a fabricationmethod of the secondary battery which includes a battery unit having apositive electrode plate, a negative electrode plate and a separatorinterposed therebetween, a can for accommodating the battery unit, a capassembly having a cap cover, a safety vent and a gasket, where the endof the safety vent is bent inwards to be filled with the gasket providedalong the outer periphery of the safety vent reportedly so that thesafety vent is inserted into the gasket in a secure manner.

U.S. Publication No. 2005/0244706 relates to an electrochemical cellwith a collector assembly for sealing the open end of a cell container.The collector assembly includes a retainer and a contact spring with aperipheral flange, each having a central opening therein. A pressurerelease vent member disposed between the retainer and the peripheralflange of the contact spring reportedly seals the openings in theretainer and contact spring under normal conditions and ruptures torelease pressure from within the cell when the internal pressure exceedsa predetermined limit.

U.S. Publication No. 2006/0228620 relates to a closure assembly andrupturable vent seal adapted for use in an electrochemical battery cell.The vent seal includes a series of peripheral projections that can befolded to insure proper sealing of the vent without wrinkles oroverlapping folded portions.

U.S. Publication No. 2007/0015046 relates to a lithium secondary batteryhaving protrusions or depressions formed on a surface of a gasket whichmakes contact with a safety vent so that gas, which is generated insidethe battery, and an electrolyte, reportedly do not leak, therebyreportedly improving safety of the battery.

U.S. Publication No. 2008/0070109 relates to a flat-shaped non-aqueouselectrolyte secondary battery that includes an electrode body formed byopposing a positive electrode and a negative electrode while interposinga separator therebetween, an outer case for housing the electrode body,and a sealing plate for sealing an opening of the outer case and an endpart of the sealing plate positioned inside the outer case. Also, thesealing plate functions as a positive electrode terminal, the outer casefunctions as a negative electrode terminal, and a surface layer of thesealing plate in contact with the positive electrode is formed with ametal layer made of aluminum or aluminum alloy.

Japanese Publication No. 09-274900 relates to a nonaqueous secondarybattery with a structure that reportedly does not cause electrolyteleakage and reportedly provides an increase of battery resistance toimpact applied.

Japanese Publication No. 10-340714 prevents breakage of an explosionproof valve body and reportedly develops this function by arranging theexplosion proof valve body on the inside of a battery more than aterminal cap and connecting an electrode lead to the explosion-proofvalve body through a welding plate.

Japanese Publication No. 2007-141673 provides a bobbin-type lithiumprimary battery whose cost can be reduced by using a nickel-plated steelplate as the material for a positive electrode can that can bereportedly used and preserved for a long period.

In view of the disclosures above, it would be desirable to provide anelectrochemical cell having a closure assembly having an end assemblythat exhibits desirable barrier properties to vapor transmission whilestill allowing emergency venting as necessary via a vent member.

SUMMARY OF THE INVENTION

In view of the above, it is an object of the present invention toprovide an electrochemical cell having a closure assembly comprising acontainer with an open end sealed by an end assembly that forms aneffective barrier to electrolyte vapor transmission and hence, massloss.

A further object of the invention is to provide an electrochemical cellhaving a container having an open end closed by an end assembly, whereinthe container imparts axial and radial forces on the end assembly toprovide leakage suppression. In a preferred embodiment, the container iscylindrical and has a circumferential bead in the sidewall thatprotrudes inwardly and upwardly towards the container opening, with thebead upper and lower walls maintained in a spaced apart relationship toallow for the container to be axially compressed between the upperinwardly crimped end of the container and the upper wall of the beadwith a portion of the end assembly located therebetween.

A further object of the present invention is to provide anelectrochemical cell having a closure assembly which provides forincreased path length for vapor transmission, wherein the path length isincreased in one embodiment by providing the container with a beadhaving a relatively deep inwardly projecting depth. The beadconfiguration can be accompanied by a seal member having a relativelysmall thickness between the bead and a component of the end assembly tominimize the cross section area of a potential electrolyte vapor outlet.

A further object of the present invention is to provide anelectrochemical cell having an end assembly comprising a vent membercapable of venting at a predetermined internal pressure, wherein thevent member is resistant to electrolyte vapor transmission and can be,for example, a foil vent or a ball vent.

Yet a further object of the present invention is to provide anelectrochemical cell having a closure assembly comprising a containerand an end assembly comprising a seal member that electrically isolatesthe sidewall from at least electrically conductive components of the endassembly having a polarity different than the container, wherein the endassembly includes an internal contact member, preferably havingspring-like characteristics, connected to a pressure release ventmember, with the contact member having a peripheral flange that acts incombination with a seal member and a container sidewall to provide bothaxial and radial sealing to the cell to reduce vapor transmission.

Still another object of the present invention is to provide a cellhaving a closure assembly including a contact member having a peripheralflange that includes two separated axial segments that aid in providingradial compression between the contact member and a container sidewall,with a seal member disposed therebetween.

An additional object of the present invention is to provide anelectrochemical cell having a closure assembly that aids in shielding aninsulating, polymeric seal member of the closure assembly fromelectrolyte, wherein the closure assembly includes a contact memberelectrically connected to an inner cover, and further including aninsulating member located between and in contact with an internalportion of a bead of the container and the contact member, with theinsulating member further functioning to prevent contact between acurrent collector of an electrode of the cell and the container whichhas a different polarity than the current collector.

Another object of the present invention is to provide a method forforming an electrochemical cell, particularly including a container foran electrochemical cell, comprising forming an elongated and upwardlytapered bead upper wall in the container sidewall in order to provideenhanced axial and radial sealing forces, which seeks to minimize massloss due to leakage of electrolyte vapor.

In one aspect of the invention, an electrochemical cell is disclosed,comprising a cylindrical metal container having a closed bottom end, asidewall and an open end, a spirally wound electrode assembly disposedwithin the container, said electrode assembly comprising a positiveelectrode, a negative electrode consisting essentially of lithium or alithium alloy, a separator disposed between the positive and negativeelectrodes, and a non-aqueous volatile electrolyte, a circumferentialinward projection in the sidewall and having an upper wall and a lowerwall connected by a transition member, the upper wall inclined upwardlytowards a radial center of the cell, and the upper wall spaced apartfrom the lower wall along their respective lengths, and an end assemblyclosing the open end of the container, the end assembly comprising avent member capable of venting at a predetermined internal pressure, acurrent limiting or interrupting member, and an insulating, polymericseal member located between the container and a conductive contact ofthe end assembly, and wherein the conductive contact is operativelyelectrically connected to the positive electrode or negative electrode.

In a further aspect of the present invention, a method for forming anelectrochemical cell is disclosed, comprising the steps of providing acylindrical container having a closed bottom end, a sidewall and an openend, forming an initial bead in the sidewall of the container afterinsertion of an electrode assembly into the container, wherein theinitial bead is located at a cell axial height above the electrodeassembly, inserting an end assembly into the container so that aperipheral portion of the end assembly is seated on an upper wall of theinitial bead, providing support to a) the bottom end of the container,b) the initial bead with a bead support, and c) the opened end of thecontainer and tapering the upper wall upward towards the radial centerof the cell, and crimping the open end of the container sidewall andsecuring the end assembly between the crimped end and a portion of theupwardly tapered upper wall to form a sealed cell, wherein the beadupwardly tapered upper wall is spaced from a lower wall of the bead inthe sealed cell.

In still another aspect of the present invention, an electrochemicalcell is disclosed, comprising a cylindrical conductive container havinga closed end, an open end sealed by an end assembly, and a sidewallextending between the closed end and the open end, the conductivecontainer being of a first polarity and the end assembly having acontact assembly of a second polarity, said sidewall having an inwardlyextending bead, an electrode assembly comprising a positive electrode, anegative electrode and a separator disposed between the electrodes, andan electrolyte, wherein one of the electrodes is in operative electricalcontact with the container and the other electrode is in operativeelectrical contact with the contact assembly of the end assembly, andthe end assembly comprising a seal member that electrically isolates thesidewall from electrically conductive components of the end assemblyhaving the second polarity, wherein the contact assembly includes aconductive contact member having a peripheral flange connected to apressure release vent member, the pressure release vent member capableof rupturing in response to internal cell pressure that is at least ashigh as a predetermined release pressure thereby allowing matter toescape through the vent member, wherein the peripheral flange includesan axial segment that extends an axial distance substantially parallelto a segment of the sidewall adjacent thereto, wherein the peripheralflange includes a radial segment extending from the axial segment in asubstantially radial direction and includes a portion located axiallyabove the bead, wherein the peripheral flange transitions from theradial segment to a second lower axial segment extending in asubstantially axial direction, and wherein the seal member is undercompression between at least (a) the sidewall and peripheral flangeaxial segment, (b) the bead and the radial segment, and (c) the bead andthe second lower axial segment.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and other features andadvantages will become apparent by reading the detailed description ofthe invention, taken together with the drawings, wherein:

FIG. 1 is a partial cross-sectional elevational view of one embodimentof an electrochemical cell of the present invention;

FIG. 2 is a cross-sectional elevational view of one embodiment of aportion of a container having an inward projection with an upper wallinclined upwardly;

FIG. 3 is a cross-sectional elevational view of a further embodiment ofa closure assembly of an electrochemical cell of the present invention;and

FIG. 4 is a cross-sectional elevational view of still another embodimentof a closure assembly of an electrochemical cell of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to electrochemical cells, preferably containinglithium or lithium alloy as an electrochemically active material and anon-aqueous electrolyte, with a cell closure assembly including acylindrical container having an open end sealed by an end assemblyincluding a pressure release vent member capable of venting when theinternal pressure of the cell is at or above a predetermined pressure.The invention will be better understood with reference to the drawings,wherein FIG. 1 illustrates one embodiment of a cylindricalelectrochemical cell 10 of the present invention. Cell 10 is a primaryFR6-type cylindrical Li/FeS₂ cell. However, it is to be understood that,as described herein, the invention is applicable to other cell types,materials and constructions.

Cell 10 has a housing 12 that includes a container 14 in the form of acan with a closed bottom and an open top end. The open top end is closedwith an end assembly 30 that cooperates with the open top end. Thecontainer 14 has a circumferential inward projection or bead 16 near thetop end of the container that supports a portion of the end assembly 30.Bead 16 is generally considered to separate the top and bottom portionsof the container 14. The closure assembly including container 14 and endassembly 30 seals an electrode assembly 60 within the bottom portion ofthe container 14. The electrode assembly 60 includes an anode ornegative electrode 62, a cathode or positive electrode 64 and aseparator 66 disposed between the negative electrode 62 and the positiveelectrode 64. Electrolyte is also disposed within the bottom portion ofthe container 14. In the illustration shown in FIG. 1, the negativeelectrode 62, positive electrode 64 and separator 66 are each relativelythin constructions which are wound together in a spiral, also known as a“jellyroll” configuration. Electrochemical cell 10, as illustrated, iscylindrical, however, one skilled in the art can appreciate thatalternative embodiments of the present invention can also include cellsand electrodes of other shapes. The container 14 can be one of severalgeometric shapes for open-ended containers, for example, prismatic andrectangular containers, provided that the teachings regarding theclosure assembly are followed. As the sealing of an open-endedcylindrical cell presents challenges regarding the radial and axialforces required to create the seal, the end assembly 30 which cooperateswith the container 14 to minimize vapor transmission is expected to haveparticular applicability to cylindrical containers.

Container 14 is preferably a metal can having an integral closed bottom.However, a metal tube that is initially open at both ends may be used insome embodiments. Container 14 in one embodiment is steel that isoptionally plated, for example, with nickel on at least the outside toprotect the exposed surface of the container from corrosion or toprovide a desired appearance. In one embodiment the container is formedusing a drawing process and can be made from a diffusion annealed, lowcarbon, aluminum killed, SAE 2006 or equivalent steel, with a grain sizeof ASTM 9 to 11 and equiaxed to slightly elongated grain shape. Othermetals may be used in alternative embodiments, for example, when theopen circuit voltage of the cell is designed to be greater than or about3 volts, or the cell is rechargeable, in order to provide relativelygreater corrosion-resistance. Examples of alternative containermaterials include, but are not limited, stainless steels, nickel platedstainless steels, nickel clad stainless steel, aluminum and alloysthereof.

As illustrated in FIGS. 1 and 2, bead 16 is an inward projectionpreferably extending circumferentially around the cylindrical container.Bead 16 has an upper wall 18, a lower wall 20 and transition member 22which connects the upper wall 18 to lower wall 20. Upper wall 18 isinclined upwardly towards the radial center of the cell, and aids inproviding a desired axial compression between the upper wall 18 of thebead 16 and the crimped end 24 of container 14. As shown in detail inFIG. 2, upper wall 18 includes a lowermost point 27 and an uppermostpoint 28 located radially closer to the center of cell 10 when comparedto the lowermost point 27. In order to provide desired axial sealingforces, and aiming to minimize weight loss due to leakage, the upperwall 18 is provided with a preferred angle of taper. More specificallyas illustrated in FIG. 2, an angle, α, exists between an imaginaryhorizontal line, i.e., perpendicular to the axial direction of the cell,extending through upper wall lowermost point 27 and an imaginary linedrawn between upper wall lowermost point 27 and upper wall uppermostpoint 28 which is at least 1°, generally from about 1° or about 2° toabout 30° and preferably from about 3° or about 5° to about 20°. Theactual contour of the upper wall surface located between lowermost point27 and uppermost point 28 can vary, and for example, can be curved orlinear. Likewise, as illustrated in FIG. 1, the cell lower wall 20 mayalso include an upwardly slanted taper extending in a direction towardthe radial center of the cell. Transition member 22 is preferablyrounded or curved and maintains a desired spacing between upper wall 18and lower wall 20. The space between the upper wall 18 and the lowerwall 20 allows insertion and removal of a support tool utilized duringthe closing process.

Bead 16 has a relatively deep depth, providing an increased path lengthfor electrolyte vapor migration, thereby slowing vapor migration. Therelatively deep bead depth also provides a desirable radial sealingforce to the cell 10. Bead depth as defined herein is measured on theexterior of container 14 as the horizontal or radial distance betweenthe greatest radially inwardly extending external surface portion oftransition member 22 and a point located on an imaginary line extendedvertically or axially from the maximum radius of upper sidewall 26.Lower sidewall 29 is situated below bead 16 as illustrated in FIG. 1.The bead is preferably formed in the container sidewall after theelectrode assembly has been placed in the lower portion of thecontainer.

In preferred embodiments, the bead depth preferably is greater than 1.5mm for a R6 size cell, greater than 1.1 mm for a R03 type cell. Stateddifferently, the bead depth is at least 22%, desirably at least 26% andpreferably 30% of the maximum radius of the cell container 14 for R6 orR03 size cells or other cell sizes.

The end assembly 30 is disposed in the top portion of container 14 andincludes a conductive contact terminal 32, optionally, a currentlimiting or interrupting member 34, a pressure release vent member 36, aseal member 40 and a contact spring or member 50 defining an opening.The end assembly 30 optionally includes a retainer 42 defining anopening. The insulating, polymeric seal member 40 is disposed between atleast the components of the end assembly 30 having a polarity differentthan the polarity of container 14 that might otherwise make contacttherewith. The current limiting or interrupting member 34, when present,is disposed in an electrical path between the conductive contactterminal 32 and the positive electrode 64 of electrode assembly 60. Theconductive contact terminal 32 preferably protrudes above the end ofcontainer 14 and is held in place by the inwardly crimped end 24 ofcontainer 14 with seal member 40 disposed therebetween and preventingelectrical contact between the two components. Conductive contactterminal 32 can be provided with one or more vent apertures 33 forallowing release of fluid if vent member 36 is breached. Retainer 42 isillustrated as a washer including an aperture through which fluids canalso pass if vent member 36 is breached or ruptured. Contact member 50is operatively electrically connected to the conductive contact terminal32 either directly or indirectly such as shown through the currentlimiting or interrupting member 34.

Contact member 50 has a shape that cooperates with seal member 40 andcontainer bead 16 and upper sidewall 26 in order to provide for adesired seal to minimize vapor transmission. The positive electrode 64of electrode assembly 60 is electrically connected to the contact member50 directly or indirectly through a lead. Contact member 50 preferablyhas at least one tab 51 in contact with an upper end of a currentcollector 65 of the positive electrode that is disposed at the top ofthe electrode assembly 60. Current collector 65 of the positiveelectrode 64 is an electrically conductive substrate, for example ametal substrate, on which the positive electrode materials are disposed,and extends beyond the positive electrode materials and the separator66. Current collector 65 may be made from any suitable material, forexample copper, aluminum, or other metals or alloys of the above so longas they are substantially stable inside the cell and compatible with thematerials utilized therein. Current collector 65 can be in the form of athin sheet, foil, a screen or expanded metal in preferred embodiments.

Contact member 50 can be made of one or more conductive materials,preferably having spring-like characteristics, for example, shape memoryalloys or bimetallic materials, although any component which makes andmaintains a sufficient electrical contact with the desired componentscan be utilized.

When the end assembly 30 is placed into container 14 during assembly,the current collector 65 is biased against tab 51 of contact member 50which, as indicated above, is resilient and/or resistant to force. Thecharacteristics of tab 51 aid in maintaining contact between contactmember 50 and current collector 65. Optionally, the tab 51 can be weldedto the current collector 65 or connected via an electrically conductivelead, such as a narrow metal strip or wire that can be welded to boththe tab 51 and current collector 65. Welded connections can sometimes bemore reliable, especially under relatively harsh handling, storage anduse conditions, but pressure connections do not require additionalassembly operations and equipment.

Contact member 50 has a peripheral flange connected to tab 51 configuredcomplimentary to the shape of the adjacent container sidewall with agoal of minimizing vapor transmission. Contact member 50 has an axialsegment 52 connected to tab 51, a radial segment 53 extending radiallyoutward from axial segment 52 and a further axial segment 54 locatedradially outwardly from radial segment 53 that transitions into aninwardly folded end 55. The lower axial segment 52 extends below theuppermost point 28 of wall 18 of bead 16 to provide for a desiredcompression force between axial segment 52 and bead 16. The two axialsegments 52 and 54 are separated by radial segment 53 and are disposedat different radial distances from each other with axial segment 52being located closer to the radial center of the cell when compared toaxial segment 54. It is to be understood that axial segments 52 and 54and radial segment 53 may not be completely linear and can havevariations in form along their respective lengths. The noted axial andradial segments, namely 52, 54 and 53, may vary along their length fromthe respective axial or radial directions at an angle up to about 45°from vertical with respect to the axial segments and horizontal withrespect to the radial segments.

The configuration of the contact member axial and radial segments allowsseal member 40 to be compressed between a) the container upper sidewall26 and the peripheral flange axial segment 54, b) a portion of the beadupper wall 18 and radial segment 53, and c) the bead 16 and axialsegment 52. The multiple radial and axial compression areas between thecontainer and the contact member with the seal member disposedtherebetween are designed to reduce the ability of electrolyte vapor toescape from the cell. The design of the contact member is one factor inallowing the thickness of the seal member to be reduced, therebyminimizing the average of cross-section area for vapor transmissionthrough the seal member. The configuration also provides for arelatively long pathway through which the electrolyte vapor must travelin order to escape from the cell. The contact member provides radial andaxial structural strength to withstand radial and axial sealing forces.In some embodiments, the contact member participates in sealing the ventmember.

Seal member 40 provides a seal between other components of end assembly30 and sidewall container 14. In one embodiment, the seal member extendsfrom below the upper wall 18 of the bead 16, preferably at least fromadjacent or below the transition member 22 and generally adjacent to aninsulating member 68 which physically separates a portion of the currentcollector 65 from the sidewall of the container below the bead 16, andup to or past the crimped end 24 of the top portion of container 14.Bead 16 provides a seating surface for the end assembly 30. Seal member40, as indicated above, physically separates at least the conductivecomponents of the end assembly from the container 14 and also seals theperipheral edges of the components of the end assembly 30 to preventcorrosion and leakage of electrolyte between these components. Sealmember 40 is sized so that upon inserting the closure assembly into thecontainer 14 and closing or crimping the top end of the container, theseal member is compressed to create a seal between the seal member andcontainer 14 as well as between the seal member 40 and the interfacialsurfaces of the other adjacent components of the end assembly 30.Initial wall thickness of the seal member can be different in one ormore locations along the path length thereof. In one embodiment, sealmember average thickness after reduction by the closing process is lessthan 0.55 mm for a R6 size cell and less than 0.37 mm for R03 and R8size cells. In a preferred embodiment, seal member 40 undergoes at leasta 10% reduction in at least one cross-sectional area upon closure of thecell, which is generally sufficient to absorb any variations in partdimensions and maintain compression under a range of conditions the cellis subjected to.

A goal of the present invention is to minimize the surface area of theseal member exposed to the electrolyte as weight loss in someembodiments can be attributed to diffusion through seal members atrelatively high temperatures. The seal member 40 is also made of amaterial composition that can form a compression seal with other cellcomponents and it also has low vapor transmission rates in order tominimize, for example, the entry of water into the cell and loss ofelectrolyte from the electrochemical cell. The seal member 40 caninclude a polymeric composition, for example, a thermoplastic orthermoset polymer, the composition of which is based in part uponfactors such as chemical compatibility with the components of theelectrode assembly, namely the negative electrode 62, positive electrode64, as well as the electrolyte, such as a non-aqueous electrolyte usedin the electrochemical cell 10. The seal member is made from anysuitable material that provides the desired sealing and insulatingproperties. Examples of suitable materials include, but are not limitedto, polypropylene, polyphenylene sulfide, tetrafluorideperfluoroalkylvinyl ether copolymer, polybutylene terephthalate, ethylenetetrafluoroethylene, polyphthalamide, or any combination thereof.Preferred gasket materials include polypropylene (e.g., PRO-FAX® 6524from Basell Polyolefins, Wilmington, Del., USA), polybutyleneterephthalate (e.g., CELANEX® PBT, grade 1600A from Ticona-U.S., Summit,N.J., USA) and polyphenylene sulfide (e.g., TECHTRON® PPS from BoedekerPlastics, Inc., Shiner, Tex., USA), and polyphthalamide (e.g., Amodel®ET 1001 L from Solvay Advanced Polymers of Alpharetta, Ga., USA). Theseal member compositions can optionally contain reinforcing fillers suchas inorganic fillers and/or organic compounds.

The seal member 40 may be coated with a sealant to further enhancesealing properties. Ethylene propylene diene terpolymer (EPDM) is asuitable sealant material, but other suitable materials can be used.

As evident from FIG. 1, the contact member 50 is designed such that itseals a relatively large percentage of surface area of the seal member40 that otherwise would be exposed to the electrolyte within the cell.The seal member thickness is small in order to provide a relativelysmall cross-section area for vapor transmission, thereby minimizing thesame.

The seal members of the invention can have a number of differentconfigurations in order to aid in meeting the goal of containing vaporswithin the cell. The seal member illustrated in FIG. 1 is formed as ahollow cylinder or annulus having various radial dimensions along itsaxial length. After closing, the seal member has an upper radial segmentsubstantially extending in a radial direction, situated below crimpedend 24 of container 14. At least a portion of the upper radial segmentis under axial compression as it is located between the bead 16 andcrimped end 24 which are axially compressed during the cell closing orsealing process. The upper radial segment transitions into an upperaxial segment substantially extending in an axial direction adjacent tothe container upper sidewall 26. The upper axial segment generallyextends between the crimped end 24 and the upper wall 18 of the bead 16.The peripheral portions of the contact terminal 32, the current limitingor interrupting member 34 and the contact member 50 are adjacent theupper axial segment, which is under radial compression between the sameand the upper sidewall 26. The seal member transitions to a lower radialsegment extending in a substantially radial direction along the upperwall 18 of bead 16. The lower radial segment has a portion that is alsounder axial compression, being located between the upper wall 26 and thecrimped end 24. The seal member 40 also has a lower axial segmentextending in a substantially axial direction from the inner end of thelower radial segment. The lower axial segment has a portion that isradially compressed between the transition member 22 of bead 16 and theaxial segment 52 of contact member 50. As illustrated, the seal memberlower axial segment is located closer to the radial center of the cellcompared to the upper axial segment.

As indicated herein, in some embodiments a major source of weight lossduring a temperature cycling test, such as a T2 test, can be electrolytevapor transmission through the seal member. According to a T2 testprocedure, test cells and batteries are stored, after determining theirinitial weight, for at least six hours at a test temperature equal to75±2° C., followed by storage for at least six hours at a testtemperature equal to −40±2° C. The maximum time interval between testtemperature extremes is 30 minutes. This procedure is repeated 10 times,after which all test cells and batteries are stored for 24 hours atambient temperature (20±5° C.) and subsequently reweighed. The weightloss is the difference between the initial weight and the post-testweight. Weight loss by diffusion through the seal member can becalculated by multiplying the vapor transmission rate by dimensionalratio of the seal member and time. Diffusion weight loss can be reducedby one or more of reducing the dimensional ratio and the vaportransmission rate. In order to reduce weight loss, the dimensional ratiocan be decreased by reducing cross section area or increasing pathlength, or a combination thereof.

For a uniform material positioned over an open end of a container filledwith electrolyte, the dimensional ratio can be calculated easily. Herethe cross-sectional area is the surface area of the membrane that isexposed to the electrolyte vapor and the path length is the membranethickness. Due to the irregular shape of seal members, calculations ofthe dimensional ratios are more difficult. When utilized in the presentinvention, finite element diffusion analysis is utilized to calculatethe dimensional ratio. In the finite element diffusion analysis, theflux integrated across a cross section bonded by sealed surfaces is thedimensional ratio if the diffusion coefficient in the seal member andvapor concentration at the seal member internal surface that is exposedto electrolyte are assumed to be unit and the vapor concentration at theseal member external surface that is exposed to the ambient is assumedto be zero. It is also assumed that the interface between a currentlimiting or interrupting member and contact member or inner cover is notsealed and, therefore, the vapor concentration at the seal membersurface adjacent to the interface is assumed to be zero. Samples ofcommercially available software that can be utilized to performdiffusion analysis modeling include MSC.MARC 2005r3 available from MSC.Software, Los Angeles, Calif. and ABAQUS available from SIMULIA,Providence, R.I. MSC.MARC was utilized to calculate the dimensionalratios presented herein. For a R6-size cell as illustrated in FIG. 1,the calculated dimensional ratio is 0.279 cm (0.110 in.). For a R6 sizelithium-iron disulfide cell constructed as disclosed in FIG. 1, thedimensional ratio of the seal member is generally less than 1.14 cm(0.45 in.), desirably less than 0.86 cm (0.34 in.) and preferably lessthan 0.51 cm (0.20 in.) Likewise, for R03 and R8 (AAAA) size cells,dimensional ratios of the seal members are generally less than 0.86 cm(0.34 in.), desirably less than 0.48 cm (0.19 in.) and preferably lessthan 0.30 cm (0.12 in.).

In a preferred embodiment of the present invention, the closing processfor forming a finished cell reduces the seal member wall thickness invarious areas. In a preferred embodiment, the smallest cross section ofthe gasket is located near the base of the gasket adjacent the inlet ofthe vapor path, for example, the portion of the seal member locatedbetween axial segment 52 of contact member 50 and transition member 22of bead 16. The cross-sectional area between the contact member and thebead is less than 12.5 mm² for a R6-size cell and less than 6.3 mm² forR03 size cells in preferred embodiments. That said, the axial segment 52of contact member 50 for a sealed R6-size cell is extended at least 0.25mm axially below upper wall uppermost point 28 of the bead 16 in oneembodiment, and preferably below the transition member 22 segment of thebead.

In the embodiment illustrated in FIG. 1, the vent member 36 is disposedin the opening defined by the peripheral flange of the contact member50. More specifically, the vent member 36 periphery is secured betweenaxial segment 53 and folded end 55 of the peripheral flange of contactmember 50. In the embodiment illustrated, retainer 42 is also securedbetween axial segment 53 and folded end 55 of contact member 50. Theseal between the vent member 36 and contact member 50 can be the resultof tight pressure contact at the interfacial surfaces, which can, insome embodiments, be enhanced by compression of the peripheral portionof the vent member 36. Optionally, an adhesive or sealant can be appliedto the desired interfacial surfaces to connect the vent member 36 tocontact member 50 and thereby form a desired seal. Axial forcesgenerated during crimping or closing of the container 14 during assemblyare also placed on the peripheral portions of the components of the endassembly 30 including the vent member 36 as illustrated in FIG. 1.

Gases are generated within the cell due to environmental conditions suchas temperature and, in certain instances, generated during normaloperation through chemical reactions. The cell contents aresubstantially contained within the electrochemical cell by the pressurerelease vent member below a predetermined pressure. The pressure releasevent member 36 periphery is compressed a sufficient amount to preventthe pressure release vent member from creeping inwardly so as to form anaperture in the opening defined by the contact member 50 when the cellinternal pressure is less than the predetermined release pressure. Whenthe pressure within the electrochemical cell is at least as high as apredetermined release pressure, the vent member 36 ruptures and allowsfluid, in the form of liquid or gas or a combination thereof, within thecell to escape through the opening in the vent member 36. The fluidwithin the cell can escape through the one or more vent apertures 33 inthe conductive contact terminal 32. The predetermined release pressurecan vary according to the chemical composition of the cell. Thepredetermined pressure is preferably above a pressure which will avoidfalse vents due to normal handling and usage or exposure to the ambientatmosphere. For example, in an FR6-type lithium-containingelectrochemical cell, the predetermined release pressure, for examplethe pressure at which the vent member 36 creates an opening, forexample, via rupturing, can range from about 10.5 kg/cm² (150 lbs/in²)to about 112.6 kg/cm² (1600 lbs/in²) and in some embodiments, from about14.1 kg/cm² (200 lbs/in²) to about 56.3 kg/cm² (800 lbs/in²) at roomtemperature, about 21° C. The pressure at which the pressure releasevent member 36 ruptures can be determined by pressurizing a cell, e.g.,through a hole punctured in the container.

As described hereinabove, the electrochemical cell 10 of the presentinvention can optionally include a current limiting or interruptingmember 34 which is disposed in the electrical path between the currentcollector 65 of the positive electrode 64 and the conductive contactterminal 32. The current limiting or interrupting member 34 can slow orprevent the continued cell internal heating and pressure build-up and/orprevent current flow, which conditions can result from electrical abusessuch as internal short circuiting, abnormal charging and forced deepdischarging. However, if the internal pressure builds to thepredetermined release pressure, the pressure release vent member 36ruptures to relieve the internal pressure. The current limiting orinterrupting member can be, for example a positive temperaturecoefficient (PTC) device or for example a thermal current interruptingswitch, such as described in U.S. Ser. No. 11/787,436, herein fullyincorporated by reference.

As indicated hereinabove, the vent member 36 can be for example, a foilvent or a ball vent.

A further embodiment of an electrochemical cell 100 of the presentinvention is illustrated in FIG. 3. Cell 100 includes an end assemblyincluding a pressure relief vent member 136 and a contact memberassembly including a conductive inner cover 151 having a vent well 137that projects downwardly away from the positive contact terminal 132,internal to the electrochemical cell 100. In one embodiment, the innercover is formed from a material as described for the contact member. Thevent well 137 has a vent aperture 138 formed therein which is sealed bythe vent ball 139 and vent bushing 141 when they are seated in vent well137 such that the bushing 141 is compressed between the vent ball 139and the vertical wall of the vent well 137. In one embodiment, the ventbushing 141 is a thermoplastic. When the internal pressure of theelectrochemical battery cell 100 exceeds a predetermined level, the ventball 139 and in some cases both the bushing 141 and the vent ball 139are forced away from the vent aperture 138 and at least partly out ofthe vent well 137 to release pressurized fluid through the vent aperture133 of cell 100. The cell illustrated in FIG. 3 further includes aconductive tab contact member 150 electrically connected to inner cover151. Inner cover has a U-shaped peripheral wall having an inner axialsegment 152 and an outer axial segment 154 of substantially the sameheight, i.e., having a difference of generally less than 20%, andpreferably less than 10%, both extending in a substantially axialdirection of the cell 100. The axial segments 152 and 154 are connectedin this embodiment by a radially extending segment 153. Theconfiguration of inner cover 151 aids in forming an electrolytemigration barrier in combination with seal member 140 and containerupper sidewall 126. As the radially extending segment 153 of inner cover151 is located above bead 116 with seal member 140 having a portionlocated therebetween, during closing of the cell, the seal member 140 isaxially compressed between bead 116 and inner cover 151. Moreover, theaxially extending segments 152 and 154 aid in providing radialcompression of seal member 140 in conjunction with the adjacent sidewall126 of the container as seal member 140 also includes a portion locatedbetween axial segment 154 and sidewall 126. Cell 100 further includes acurrent limiting or interrupting member 134 disposed between innerrollback cover 151 and contact terminal 132. Furthermore, the cellillustrated in FIG. 3 includes an inwardly projecting bead 116 having anupper wall 118 inclined upwardly towards the radial center of the cell,as described hereinbelow with respect to FIG. 1, and further includestransition segment 122 and lower wall 120.

The seal member 140 illustrated in FIG. 3 is formed as an annulus havinga “C”-shaped vertical cross section. After closing, the seal member hasan upper radial segment substantially extending in a radial direction,situated below crimped end of upper sidewall 126 of container 114. Atleast a portion of the upper radial segment is under axial compressionas it is located between the bead 116 and the crimped end which areaxially compressed during the cell closing or sealing process. The upperradial segment transitions into an outer axial segment substantiallyextending in an axial direction adjacent to the container upper sidewall126. The outer axial segment generally extends between the crimped endand the upper wall 118 of the bead 116. The peripheral portions of thecontact terminal 132, the current limiting or interrupting member 134and the inner cover 151 are adjacent the outer axial segment, which isunder radial compression between the same and the upper sidewall 126.The seal member outer axial segment transitions to a lower radialsegment extending in a substantially radial direction along the upperwall 118 of bead 116. The lower radial segment has a portion that isalso under axial compression, being located between the upper wall 118and the crimped end of sidewall 126. The seal member transitionsupwardly from the lower radial segment into an inner axial segment thatextends a distance upwardly between the inner axial segment 152 of innercover 151 and the contact member 150.

In order to provide an additional hindrance to electrolyte migrationthrough seal member 140, seal member 140 is shielded from the internalportion of the cell containing the electrode assembly and electrolyte bythe inner cover 151, contact member 150 and insulating member 168.Insulating member 168 has a dual purpose of providing a portion of abarrier to electrolyte migration as indicated, as well as to prevent thecurrent collector of the electrode electrically connected to contactterminal 132 from contacting the sidewall of container 114. In apreferred embodiment as illustrated, a portion of insulating member 168is disposed between and in contact with both bead 116 and contact member150, thereby forming an additional seal to impede or slow electrolytemigration.

A further embodiment of an electrochemical cell having a ball vent isillustrated in FIG. 4. While FIG. 4 shows an upper portion of cell 200,a lower portion of the cell can be similar to that shown in FIG. 1. Cell200 includes a pressure relief vent member 236, a ball vent. Cell 200includes a conductive contact member assembly including an inner cover251 having a vent well 237 that projects downwardly away from positivecontact terminal 232. Vent well 237 has a vent aperture 238 formedtherein which is sealed by vent ball 239 and vent bushing 241, when theyare seated in vent well 237 such that the bushing 241 is compressedbetween the vent ball 239 and the vertical wall of vent well 237. Asindicated hereinabove, when the internal pressure of the cell 200exceeds a predetermined level, the pressure relief vent member 236allows venting through vent aperture 238 and further through ventaperture 233 of the contact terminal 232. The cell further includes acurrent limiting or interrupting member 234.

Cell 200 further includes a tab-type contact member 250 electricallyconnected to inner cover 251 for contact with the current collector ofthe positive electrode. Inner cover 251 has a peripheral flange locatedradially outward from the portion of the inner cover 251 that forms ventwell 237. The peripheral flange cooperates with seal member 240 andcontainer 214 to provide an electrolyte migration barrier and adequatecell seal. Seal member 240 has a configuration similar to seal member 40shown in FIG. 1 and described herein above. The peripheral flangeincludes a radially extending segment 253 and axially extending segment252. Seal member 240 is radially compressed between a portion of axialsegment 252 and the bead 216, specifically transition segment 222 ofbead 216 as well as between the end of radial segment 253 and uppersidewall 226 of the container 214. A zone of axial compression of sealmember 240 is formed between the upper wall 218 of bead 216 and theradial segment 253 of inner cover 251. The bead 216 of cell 200, asillustrated, is an inwardly projecting bead having upper wall 218inclined upwardly towards the radial center of the cell as describedherein with respect to FIG. 1. Upper wall 218 is spaced from lower wall220 in order to impart desired axial compression to the closure assemblyof cell 200.

The vent bushing is made from a thermoplastic material that is resistantto cold flow at high temperatures (e.g., 75° C.). The thermoplasticmaterial comprises a base resin such as ethylene-tetrafluoroethylene,polybutylene terephthlate, polyphenylene sulfide, polyphthalamide,ethylene-chlorotrifluoroethylene, chlorotrifluoroethylene,perfluoroalkoxy-alkane, fluorinated perfluoroethylene polypropylene andpolyetherether ketone. Ethylene-tetrafluoroethylene copolymer (ETFE),polyphenylene sulfide (PPS), polybutylene terephthalate (PBT) andpolyphthalamide are preferred. The resin can be modified by adding athermal-stabilizing filler to provide a vent bushing with the desiredsealing and venting characteristics at high temperatures. The bushingcan be injection molded from the thermoplastic material. TEFZEL® HT2004(ETFE resin with 25 weight percent chopped glass filler) is a preferredthermoplastic material.

The vent ball can be made from any suitable material that is stable incontact with the cell contents and provides the desired cell sealing andventing characteristic. Glasses or metals, such as stainless steel, canbe used.

The pressure release vent member 36, which in the embodiment shown inFIG. 1, is a foil-type vent, disposed between the retainer 42 and thecontact member 50 includes at least one layer of a composition of metal,polymer, or mixtures thereof. It is also possible that the pressurerelease vent member 36 can include two or more layers of differentmaterial compositions. For example, a second layer having a differentcomposition than a first layer may be used for purposes of bonding thepressure release vent member 36 to the retainer 42 or to the contactmember 50. In another example, a second and a third layer having adifferent composition than the first layer may be used to bond thepressure release vent member 36 to both the retainer 42 and the contactmember 50. Also, multiple layers having two or more compositions can beused for tailoring the performance properties, for example, strength andflexibility, of the pressure release vent member 36. Ideally, separatelayers would be provided on the basis of compatibility with theelectrolyte, ability to prevent vapor transmission and/or ability toimprove the sealing characteristics of the vent member 36 within the endassembly. For example, an adhesive activated by pressure, ultrasoundand/or heat, such as a polymer or any other known material in theadhesive field that is compatible with the elements disclosed herein,could be provided as a layer of the vent member 36 in order to bond thevent member within the end assembly.

Compositions suitable for use in the foil-type pressure release ventmember 36 can include, but are not limited to, metals such as aluminum,copper, nickel, stainless steel and alloys thereof; and polymericmaterials such as polyethylene, polypropylene, polybutyleneterephthalate (PBT), polyethylene terephthalate (PET, ethylene acrylicacid, ethylene methacrylic acid, polyethylene methacrylic acid, andmixtures thereof. The composition of the pressure release vent member 36can also include polymers reinforced with metal, as well as a singlelayer or a multi-layer laminate of metals or polymers or both. Forexample, the single layer can be a metal foil, preferably aluminum foil,that is substantially impermeable to water, carbon dioxide andelectrolyte, or a non-metallized film of a polymer coated with a layerof oxidized material that prevents vapor transmission, such as, forexample SiO_(x) or Al₂O_(x). The pressure release vent member 36 canfurthermore contain an adhesive layer that contains a contact-bondingadhesive material, for example polyurethane, or a heat, pressure and/orultrasonically activated material, for example low density polyolefins.Alternatively, these or other adhesives or sealant materials can beseparately applied to a portion of the pressure release vent member(e.g., the outer periphery coming into contact with retainer 42 and/orspring 50), the retainer 42, the spring 50 or any combination thereoffor enhancing the seal within the collector assembly. A preferredlaminar vent construction would have four layers consisting of orientedpolypropylene, polyethylene, aluminum foil and low density polyethylene.

Regardless of the composition, the pressure release vent member 36should be chemically resistant to the electrolyte contained in the cell10 and should have a low vapor transmission rate (VTR) to provide a lowrate of weight loss for the cell 10 over a broad range of ambienttemperatures. For example, if the pressure release vent member 36 ismetal which is impervious to vapor transmission, the VTR through thethickness of the pressure release member 36 is substantially zero.However, the pressure release vent member 36 can include at least onelayer of vapor-permeable material, for example polymeric materials, asdescribed above, that can function, for example, as an adhesive or as anelastomeric layer to achieve a seal between the pressure release ventmember 36 and at least one of the retainer 42 and the contact member 50.

The predetermined release pressure, or the pressure at which thepressure release vent member 36 is intended to rupture, is a function ofits physical properties (e.g., strength), its physical dimensions (e.g.,thickness) and the area of the opening defined by the retainer 42 andthe opening defined by the PTC device, whichever is smaller. The greaterthe exposed area of the pressure release vent member 36, the lower thepredetermined release pressure will be due to the greater collectiveforce exerted by the internal gases of the electrochemical battery cell10. Consequently, adjustments may be made to any of these variables inorder to engineer an end assembly with a vent member without departingfrom the principles of the invention.

Depending upon the exposed area of the vent member 36 relative to theopening defined by the retainer 42, the thickness of the pressurerelease vent member can be less than about 0.254 mm (0.010 inch), and insome embodiments can range from about 0.0254 mm (0.001 inch) to about0.127 mm (0.005 inch), and in yet other embodiments the thickness canrange from about 0.0254 mm (0.001 inch) to about 0.05 mm (0.002 inch).The composition and thickness of the pressure release vent member 36 canbe determined by those of ordinary skill in the art, in view of thevapor transmission rate (VTR) and predetermined release pressurerequirements.

The pressure release vent member can include at least one layer of acomposition containing metal, polymer, and mixtures thereof. A suitablethree-layer laminate that can be used for the pressure release ventmember is PET/aluminum/EAA copolymer available as LIQUIFLEX® Grade 0539635C-501C from Curwood of Oshkosh, Wis., USA. A suitable four layermaterial of oriented PP/PE/aluminum/LDPE is FR-2175 from Ludlow CoatedProducts of Columbus, Ga., USA, which is a wholly-owned subsidiary ofTyco International, Ltd. of Princeton, N.J., USA. A suitable five-layerlaminate is PET/PE/Aluminum/PE/LL-DPE available as BF-48 also fromLudlow Coated Products of Columbus, Ga, USA. However, as noted above,any combination of laminates for polypropylene, polyethylene,non-metallized polymeric films coated with a layer of oxidized materialthat prevents vapor transmission (for example, SiO_(x) or Al₂O_(x))and/or aluminum-based foils are also specifically contemplated.

The negative electrode comprises a strip of lithium metal, sometimesreferred to as lithium foil. The composition of the lithium can vary,though for battery grade lithium the purity is always high. The lithiumcan be alloyed with other metals, such as aluminum, to provide thedesired cell electrical performance. Battery grade lithium-aluminum foilcontaining 0.5 weight percent aluminum is available from Chemetall FooteCorp., Kings Mountain, N.C., USA.

The negative electrode may have a non-consumable current collector insome embodiments, within or on the surface of the metallic lithium. Asin the cell in FIG. 1, a separate current collector may not be needed,since lithium has a high electrical conductivity, but a currentcollector may be included, e.g., to maintain electrical continuitywithin the negative electrode during discharge, as the lithium isconsumed. When the negative electrode includes a non-consumable currentcollector, it may be made of copper because of its conductivity, butother conductive metals can be used as long as they are stable insidethe cell.

In a preferred embodiment, the anode or negative electrode is free of aseparate current collector and the one or more strips or foil of lithiummetal or lithium-containing alloy solely serve as a current collectordue to the relatively high conductivity of the lithium orlithium-containing alloy. By not utilizing a current collector, morespace is available within the container for other components, such asactive materials. Providing a cell without a negative electrode currentcollector can also reduce cell cost. Preferably a single layer or stripof lithium or a lithium-containing alloy is utilized as the negativeelectrode.

An electrical lead preferably connects the anode or negative electrodeto the cell container. This may be accomplished embedding an end of thelead within a portion of the negative electrode or by simply pressing aportion such as an end of the lead onto the surface of the lithium foil.The lithium or lithium alloy has adhesive properties and generally atleast a slight, sufficient pressure or contact between the lead andelectrode will weld the components together. In one preferredembodiment, the negative electrode is provided with a lead prior towinding into a jelly-roll configuration. For example, during production,a band comprising at least one negative electrode consisting of alithium or lithium alloy is provided at a lead connecting stationwhereat a lead is welded onto the surface of the electrode at a desiredlocation. The tabbed electrode is subsequently processed so that thelead is coined, if desired, in order to shape the free end of the leadnot connected to the electrode. Subsequently, the negative electrode iscombined with the remaining desired components of the electrodeassembly, such as the positive electrode and separator, and wound into ajelly-roll configuration. Preferably after the winding operation hasbeen performed, the free negative electrode lead end is furtherprocessed, by bending into a desired configuration prior to insertioninto the cell container.

The electrically conductive negative electrode lead has a sufficientlylow resistance in order to allow sufficient transfer of electricalcurrent through the lead and have minimal or no impact on service lifeof the cell. The desired resistance can be achieved by increasing thewidth and the thickness of the tab.

The positive electrode is generally in the form of a strip thatcomprises a current collector and a mixture that includes one or moreelectrochemically active materials, usually in particulate form. Irondisulfide (FeS₂) is a preferred active material. In a Li/FeS₂ cell theactive material comprises greater than 50 weight percent FeS₂. Thepositive electrode can also contain one or more additional activematerials, depending on the desired cell electrical and dischargecharacteristics. The additional active positive electrode material maybe any suitable active positive electrode material. Examples includeBi₂O₃, C₂F, CF_(x), (CF)_(n), CoS₂, CuO, CuS, FeS, FeCuS₂, MnO₂,Pb₂Bi₂O₅ and S. More preferably, the active material for a Li/FeS₂ cellpositive electrode comprises at least 95 weight percent FeS₂, yet morepreferably at least 99 weight percent FeS₂, and most preferably FeS₂ isthe sole active positive electrode material. FeS₂ having a purity levelof at least 95 weight percent is available from Washington Mills, NorthGrafton, Mass., USA; Chemetall GmbH, Vienna, Austria; and Kyanite MiningCorp., Dillwyn, Va., USA.

In addition to the active material, the positive electrode mixturecontains other materials. A binder is generally used to hold theparticulate materials together and adhere the mixture to the currentcollector. One or more conductive materials such as metal, graphite andcarbon black powders may be added to provide improved electricalconductivity to the mixture. The amount of conductive material used canbe dependent upon factors such as the electrical conductivity of theactive material and binder, the thickness of the mixture on the currentcollector and the current collector design. Small amounts of variousadditives may also be used to enhance positive electrode manufacturingand cell performance. The following are examples of active materialmixture materials for Li/FeS₂ cell positive electrodes. Graphite: KS-6and TIMREX® MX15 grades synthetic graphite from Timcal America,Westlake, Ohio, USA. Carbon black: Grade C55 acetylene black fromChevron Phillips Company LP, Houston, Tex., USA. Binder:ethylene/propylene copolymer (PEPP) made by Polymont Plastics Corp.(formerly Polysar, Inc.) and available from Harwick StandardDistribution Corp., Akron, Ohio, USA; non-ionic water solublepolyethylene oxide (PEO): POLYOX® from Dow Chemical Company, Midland,Mich., USA; and G1651 grade styrene-ethylene/butylenes-styrene (SEBS)block copolymer from Kraton Polymers, Houston, Tex. Additives: FLUO HT®micronized polytetrafluoroethylene (PTFE) manufactured by Micro PowdersInc., Tarrytown, N.Y., USA (commercially available from Dar-Tech Inc.,Cleveland, Ohio, USA) and AEROSIL® 200 grade fumed silica from DegussaCorporation Pigment Group, Ridgefield, N.J.

The current collector may be disposed within or imbedded into thepositive electrode surface, or the positive electrode mixture may becoated onto one or both sides of a thin metal strip. Aluminum is acommonly used material. The current collector may extend beyond theportion of the positive electrode containing the positive electrodemixture. This extending portion of the current collector can provide aconvenient area for making contact with the electrical lead connected tothe positive terminal. It is desirable to keep the volume of theextending portion of the current collector to a minimum to make as muchof the internal volume of the cell available for active materials andelectrolyte.

A preferred method of making FeS₂ positive electrodes is to roll coat aslurry of active material mixture materials in a highly volatile organicsolvent (e.g., trichloroethylene) onto both sides of a sheet of aluminumfoil, dry the coating to remove the solvent, calender the coated foil tocompact the coating, slit the coated foil to the desired width and cutstrips of the slit positive electrode material to the desired length. Itis desirable to use positive electrode materials with small particlesizes to minimize the risk of puncturing the separator. For example,FeS₂ is preferably sieved through a 230 mesh (62 μm) screen before use.

The separator is a thin microporous membrane that is ion-permeable andelectrically nonconductive. It is capable of holding at least someelectrolyte within the pores of the separator. The separator is disposedbetween adjacent surfaces of the negative electrode and positiveelectrode to electrically insulate the electrodes from each other.Portions of the separator may also insulate other components inelectrical contact with the cell terminals to prevent internal shortcircuits. Edges of the separator often extend beyond the edges of atleast one electrode to insure that the negative electrode and positiveelectrode do not make electrical contact even if they are not perfectlyaligned with each other. However, it is desirable to minimize the amountof separator extending beyond the electrodes.

To provide good high power discharge performance it is desirable thatthe separator have the characteristics (pores with a smallest dimensionof at least 0.005 μm and a largest dimension of no more than 5 μmacross, a porosity in the range of 30 to 70 percent, an area specificresistance of from 2 to 15 ohm-cm² and a tortuosity less than 2.5)disclosed in U.S. Pat. No. 5,290,414, issued Mar. 1, 1994, and herebyincorporated by reference.

Suitable separator materials should also be strong enough to withstandcell manufacturing processes as well as pressure that may be exerted onthe separator during cell discharge without tears, splits, holes orother gaps developing that could result in an internal short circuit. Tominimize the total separator volume in the cell, the separator should beas thin as possible, preferably less than 25 μm thick, and morepreferably no more than 22 μm thick, such as 20 μm or 16 μm. A hightensile stress is desirable, preferably at least 800, more preferably atleast 1000 kilograms of force per square centimeter (kgf/cm²). For anFR6 type cell the preferred tensile stress is at least 1500 kgf/cm² inthe machine direction and at least 1200 kgf/cm² in the transversedirection, and for a FR03 type cell the preferred tensile strengths inthe machine and transverse directions are 1300 and 1000 kgf/cm²,respectively. Preferably the average dielectric breakdown voltage willbe at least 2000 volts, more preferably at least 2200 volts and mostpreferably at least 2400 volts. The preferred maximum effective poresize is from 0.08 μm to 0.40 μm, more preferably no greater than 0.20μm. Preferably the BET specific surface area will be no greater than 40m²/g, more preferably at least 15 m²/g and most preferably at least 25m²/g. Preferably the area specific resistance is no greater than 4.3ohm-cm², more preferably no greater than 4.0 ohm-cm², and mostpreferably no greater than 3.5 ohm-cm². These properties are describedin greater detail in U.S. patent application Ser. No. 10/719,425, filedon Nov. 21, 2003, which is hereby incorporated by reference.

Separator membranes for use in lithium batteries are often polymericseparators made of polypropylene, polyethylene or ultrahigh molecularweight polyethylene, with polyethylene being preferred. The separatorcan be a single layer of biaxially oriented microporous membrane, or twoor more layers can be laminated together to provide the desired tensilestrengths in orthogonal directions. A single layer is preferred tominimize the cost. Suitable single layer biaxially oriented polyethylenemicroporous separator is available from Tonen Chemical Corp., availablefrom EXXON Mobile Chemical Co., Macedonia, N.Y., USA. Setela F20DHIgrade separator has a 20 μm nominal thickness, and Setela 16MMS gradehas a 16 μm nominal thickness.

The negative electrode, positive electrode and separator strips arecombined together in an electrode assembly. The electrode assembly maybe a spirally wound design, such as that shown in FIG. 1, made bywinding alternating strips of positive electrode, separator, negativeelectrode and separator around a mandrel, which is extracted from theelectrode assembly when winding is complete. At least one layer ofseparator and/or at least one layer of electrically insulating film(e.g., polypropylene) is generally wrapped around the outside of theelectrode assembly. This serves a number of purposes: it helps hold theassembly together and may be used to adjust the width or diameter of theassembly to the desired dimension. The outermost end of the separator orother outer film layer may be held down with a piece of adhesive tape orby heat sealing. The negative electrode can be the outermost electrode,as shown in FIG. 1, or the positive electrode can be the outermostelectrode. Either electrode can be in electrical contact with the cellcontainer, but internal short circuits between the outmost electrode andthe side wall of the container can be avoided when the outermostelectrode is the same electrode that is intended to be in electricalcontact with the can.

In one or more embodiments of the present invention, the electrodeassembly is formed with the positive electrode having electrochemicallyactive material selectively deposited thereon for improved service andmore efficient utilization of the electrochemically active material ofthe negative electrode. Non-limiting examples of selectively depositedconfigurations of electrochemically active material on the positiveelectrode and further, an electrochemical cell, including a positivecontainer, are set forth in U.S. Publication No. 2008/0026288, publishedon Jan. 31, 2008 and U.S. Publication No. 2008/0026293, published onJan. 31, 2008, both fully herein incorporated by reference.

Rather than being spirally wound, the electrode assembly may be formedby folding the electrode and separator strips together. The strips maybe aligned along their lengths and then folded in an accordion fashion,or the negative electrode and one electrode strip may be laidperpendicular to the positive electrode and another electrode strip andthe electrodes alternately folded one across the other (orthogonallyoriented), in both cases forming a stack of alternating negativeelectrode and positive electrode layers.

The electrode assembly is inserted into the housing container. In thecase of a spirally wound electrode assembly, whether in a cylindrical orprismatic container, the major surfaces of the electrodes areperpendicular to the side wall(s) of the container (in other words, thecentral core of the electrode assembly is parallel to a longitudinalaxis of the cell). Folded electrode assemblies are typically used inprismatic cells. In the case of an accordion-folded electrode assembly,the assembly is oriented so that the flat electrode surfaces at oppositeends of the stack of electrode layers are adjacent to opposite sides ofthe container. In these configurations the majority of the total area ofthe major surfaces of the negative electrode is adjacent the majority ofthe total area of the major surfaces of the positive electrode throughthe separator, and the outermost portions of the electrode majorsurfaces are adjacent to the side wall of the container. In this way,expansion of the electrode assembly due to an increase in the combinedthicknesses of the negative electrode and positive electrode isconstrained by the container side wall(s).

A nonaqueous electrolyte, containing water only in very small quantitiesas a contaminant (e.g., no more than about 500 parts per million byweight, depending on the electrolyte salt being used), is used in thepreferred electrochemical cells of the invention. Any nonaqueouselectrolyte suitable for use with lithium and active positive electrodematerial may be used. The electrolyte contains one or more electrolytesalts dissolved in an organic solvent. For a Li/FeS₂ cell, examples ofsuitable salts include lithium bromide, lithium perchlorate, lithiumhexafluorophosphate, potassium hexafluorophosphate, lithiumhexafluoroarsenate, lithium trifluoromethanesulfonate and lithiumiodide; and suitable organic solvents include one or more of thefollowing: dimethyl carbonate, diethyl carbonate, methylethyl carbonate,ethylene carbonate, propylene carbonate, 1,2-butylene carbonate,2,3-butylene carbonate, methyl formate, γ-butyrolactone, sulfolane,acetonitrile, 3,5-dimethylisoxazole, n,n-dimethyl formamide and ethers.The salt/solvent combination will provide sufficient electrolytic andelectrical conductivity to meet the cell discharge requirements over thedesired temperature range. Ethers are often desirable because of theirgenerally low viscosity, good wetting capability, good low temperaturedischarge performance and good high rate discharge performance. This isparticularly true in Li/FeS₂ cells because the ethers are more stablethan with MnO₂ positive electrodes, so higher ether levels can be used.Suitable ethers include, but are not limited to acyclic ethers such as1,2-dimethoxyethane, 1,2-diethoxyethane, di(methoxyethyl)ether,triglyme, tetraglyme and diethyl ether; and cyclic ethers such as1,3-dioxolane, tetrahydrofuran, 2-methyl tetrahydrofuran and3-methyl-2-oxazolidinone.

Specific negative electrode, positive electrode and electrolytecompositions and amounts can be adjusted to provide the desired cellmanufacturing, performance and storage characteristics, as disclosed inU.S. patent application Ser. No. 10/719,425, which is referenced above.

Methods for assembly of the electrochemical cells of the presentinvention include inserting the electrode assembly and preferably aninsulating member into the cell container. An initial bead is formed inthe sidewall of container. The bead is formed in one embodiment bypressing a forming wheel against the sidewall of the container in thearea it is desired to form the bead while the can is rotated around itsaxial axis. Electrolyte is dispensed into the container prior toinsertion of the end assembly into container, when a foil vent isutilized. Alternatively, if a ball vent is utilized in end assembly, theelectrolyte can be added prior to internal sealing of the cell with theball of the ball vent. The peripheral portions of the end assembly areseated on the upper wall of the initial bead formed. In a further step,the container is supported at the initial bead. The bead support has aprotruding ledge that is inserted into the bead. The bead support in oneembodiment consists of two halves and each extends preferably 180°around the bead so that the bead is fully supported around thecircumference of the cell and the support can be opened and closed inthe process. The container is also supported at the bottom of thecontainer. Various seal member surfaces, both radial and axial, i.e.,perpendicular to the radial direction, are advantageously sealed againstother adjacent components of the closure assembly during the closingprocess. A further step of closing the cell and forming the upwardlyinclined bead involves the diameter reduction of the upper sidewall by aredraw or collet process. In this process, the container is constrainedor supported both at the top end and the bottom. In some embodiments,the support at the container bottom can lift upward during the processof diameter reduction. The diameter reduction and bottom lifting causeextra material flow into the bead that is further deformed or workedradially inwardly and the bead forms the desired upwardly inclinedinwardly projecting upper wall. After diameter reduction, the upper endof the container is also folded inwardly to form a crimped end and axialforces are applied between the bead and crimped end. In the crimpingprocess, the container is also supported at the bottom. Radialcompression is preferably maintained on at least the upper sidewallduring crimping of the upper end of the container.

The result of the cell forming and closing processes are illustrated inthe drawings. Geometries of the parts and the closing processes insurethat the desired interfaces between the seal member and the container,seal member and the current limiting or interrupting member, and theseal member and the contact member or inner cover outer diameter are allsealed. The contact member and/or inner cover are designed such that itseals a large portion of the seal member surface area that otherwisewould be exposed to electrolyte. The gasket thickness is minimized. Therelatively deep bead depth and relatively small seal member thicknessaid in minimizing vapor transmission and increase the path length forvapor transmission. The circumferential upwardly tapered inward beadprojection provides enhanced sealing force and further reduces thechance of electrochemical cell leakage. This is especially true when thecell is exposed to temperature fluctuation causing expansion andcontraction of cell components, especially in the polymeric seal member.The upwardly tapered inward bead upper wall has more tendency to springback to its original position after the expansion.

The above description is particularly relevant to cylindrical Li/FeS₂cells, such as FR6 and FR03 types, as defined in International StandardsIEC 60086-1 and IEC 60086-2, published by the InternationalElectrotechnical Commission, Geneva, Switzerland. However, the inventionmay also be adapted to other cell sizes and shapes and to cells withother electrode assembly, housing, seal and pressure relief vent memberdesigns. Other cell types in which the invention can be used includeprimary and rechargeable nonaqueous cells, such as lithium/manganesedioxide and lithium ion cells. The electrode assembly configuration canalso vary. For example, it can have spirally wound electrodes, asdescribed above, folded electrodes, or stacks of strips (e.g., flatplates). The cell shape can also vary, to include cylindrical andprismatic shapes, for example. Other cell chemistries such as, but notlimited to, Li/SO₂, Li/AgCl, Li/V₂O₅, Li/MnO₂, Li/Bi₂O₃ can be utilized.These batteries could have a nominal voltage higher than 1.50 V such as2.0 V and 3.0 V.

It will be understood by those who practice the invention and thoseskilled in the art that various modifications and improvements may bemade to the invention without departing from the spirit of the disclosedconcepts. The scope of protection afforded is to be determined by theclaims and by the breadth of interpretation allowed by law.

1. A method for forming an electrochemical cell, comprising the stepsof: providing a cylindrical container having a closed bottom end, asidewall and an open end; forming an initial bead in the sidewall of thecontainer after insertion of an electrode assembly into the container,wherein the initial bead is located at a cell axial height above theelectrode assembly; inserting an end assembly into the container so thata peripheral portion of the end assembly is seated on an upper wall ofthe initial bead; providing support to a) the bottom end of thecontainer, b) the initial bead with a bead support, and c) the open endof the container and tapering the upper wall upward towards the radialcenter of the cell; and crimping the open end of the container sidewalland securing the end assembly between the crimped end and a portion ofthe upwardly tapered upper wall to form a sealed cell, wherein the beadupwardly tapered upper wall is spaced from a lower wall of the bead inthe sealed cell.
 2. The method according to claim 1, wherein the beadsupport extends around the circumference of the cell during formation ofthe upwardly tapered upper wall.
 3. The method according to claim 2,wherein forming the initial bead includes pressing a forming wheelagainst the sidewall of the container while the can is rotated aroundits axial axis.
 4. The method according to claim 1, wherein during thecrimping process, the container is also supported at the bottom of thecontainer.
 5. The method according to claim 1, wherein an upper sidewalllocated above the bead is subjected to diameter reduction.
 6. The methodaccording to claim 5, wherein radial compression is maintained on atleast the upper sidewall during crimping of the upper end of thecontainer.